Solid metal molding

ABSTRACT

THE SUPERPLASTIC EUTECTOID ALLOY OF ZINC AND ALUMINUM IS DEFORMED IN A DIE HAVING A TEMPERATURE SUBSTANTIALLY IN EXCESS OF THE CRITICAL TEMPERATURE LIMIT FOR THE ALLOY WITHOUT DETERMENTAL EFFECT. AFTER FORMING, THE TEMPERATURE OF THE RESULTING PART EXCEEDS THE CRITICAL LIMIT AND TO THUS ENABLE THE PART TO BE HANDLED WITHOUT DISTORTION. IN ADDITION, THE PART TEMPERATURE AFTER FORMING IS SUFFICIENTLY HIGH TO PERMIT IMMEDIATE HEAT TREATMENT FOR ROOM TEMPERATURE STRUCTURAL PROPERTY ENHANCEMENT AS PART OF THE FORMING CYCLE SIMPLY BY CONTROLLING THE COOLING RATE OF THE PART.

y 11, 1971 D. L. mp -ML 3,578511 SOLID METAL MOLDING File d Dec. 13,1968 3 Sheets-Sheet 1 INVENTORS.

I DANIEL l.v MEHl DELBERI 1 WILSON RICHARD J. YOUNG ATTORNEY.

May 11,1971 L. MEHL, ETAL SOLID METAL MOLDING Filed Dec. v 13, 1968 3Sheets-Sheet 2 uoum T u nF I --532F 5o0-- I a h? i 10o JV 9'0 8'0 1'0WT. PERCENTAGE zmc r Zn-Al ALLOY FIG. s &5

May 11, 971 0. I... MEHL. ETAL SOLID METAL MOLDING 3 Sheets-Sheet 5Filed Dec. 13, 1968 United States Patent l 3,578,511 SOLID METAL MOLDINGDaniel L. Mehl, Lexington, Ky., Delbert T. Wilson,

Austin, Tex., and Richard J. Young, Lexington, Ky.,

assignors to International Business Machines Corporation, Armonk, NY.

Filed Dec. 13, 1968, Ser. No. 783,675 Int. Cl. C22f 1 I16 US. Cl.148-115 8 Claims ABSTRACT OF THE DISCLOSURE The superplastic eutectoidalloy of zinc and aluminum is deformed in a die having a temperaturesubstantially in excess of the critical temperature limit for the alloyWithout detrimental effect. After forming, the temperature of theresulting part exceeds the critical limit and to thus enable the part tobe handled without distortion. In addition, the part temperature afterforming is sufficiently high to permit immediate heat treatment for roomtemperature structural property enhancement as part of the forming cyclesimply by controlling the cooling rate of the part.

DISCLOSURE OF THE INVENTION Cross-references US. Pat. 3,340,101describes a process for forming materials conditioned to exhibit asignificant strain rate sensitivity at elevated temperature. The abilityof the zinc-aluminum eutectoid and other socalled superplastic materialsto be severely deformed by anomalously low applied stress has beenemployed to advantage in other processes as described for example in US.patent application Ser. No. 653,396, entitled Injection Molding of SolidMetal, filed July 14, 1967, by L. Hymes and D. L. Mehl; US. patentapplication Ser. No. 689,823, entitled Machine Assembly, filed Dec. 12,,1967 by L. Hymes; and US. patent application Ser. No. 744,843, entitledMethod of Molding Vertical Bosses, filed July 15, 1968, by D. T. Wilsonand R. J. Young.

The anomalous superplastic or low flow stress behavior in metal isusually encountered in association with a gross retransformation of ametallurgical structure from an artificially constrained and henceunstable or metastable condition. Most commonly, gross metallurgicalstructural transformations the induced by heat energy and occur atspecific energy levels or temperatures. A com mon example of a typicalso-called superplastic material is the 78% zinc-22% aluminum eutectoidwhich is prepared for superplastic behavior by quenching from a uniformtemperature of about 600 degrees F. At the time of forming, conditioningof the material is completed by raising its temperature to between about520 F. and the eutectoid invariant or 532 F. for the forming process.

The temperature of the eutectoid invariant has been demonstrated as acritical upper limit for obtaining superplastic behavior. Properlyprepared material will lose its ability to exhibit the anomoloussuperplastic behavior if it is heated above its critical limit.Historically, steps have been taken to insure that the material beingformed not exceed the eutectoid invariant. These steps principally haveinvolved the use of sophisticated temperature controls on the dies andmetal handling equipment to maintain a temperature as close aspractically possible to the eutectoid invariant but always to the lowerside. We have discovered that on a dynamic basis, superplastic materialswhich have solid phases above their critical forming temperature will berelatively tolerant of localized applied super-critical temperatures,due to the absorption of heat Patented May 11, 1971 energy by theprocess of metallurgical structure transformation occurring at thecritical temperature. We have further discovered that this tolerance canbe employed to great advantage by intelligent selection of moldingspeed, die temperature, die designs and die materials as guided by wellknown principles of heat transfer.

Accordingly, it has been an object of our invention to decrease theoverall forming cycle time for superplastic materials.

A further object of our invention has been to provide a molding processfor superplasticity material wherein the superplastic behavior issubstantially eliminated immediately following complete part formationto enhance the handle-ability of the formed part.

Another important object of our invention has been to provide a moldingprocess for superplastic materials and particularly the zinc-aluminumeutectoid wherein the process of molding is combined with a heattreatment process for enhancement of room temperature properties wherebythe overall molding and heat treatment cycle time is reduced.

A further important object of our invention has been to provide aprocess for molding superplastic metals wherein accurate control ofmetal temperature at the level of maximum formability is obtainable byreliance on the dynamics of a predictable heat transfer situation ratherthan on the accuracy of an elaborate temperature control mechanism.

An additional object of our invention has been to improve thedimensional stability of parts formed by our process by obtaining a moreuniform structure of the part before it is removed from the shaping die.

Our process involves the primary steps of:

(1) A prepared body of potentially superplastic metal is provided of acomposition known to have a wholly solid phase above its criticalsuperplastic temperature.

(2) A precision mold cavity or die is provided preferably of a materialhaving a significantly loiwer coeflicient of heat conductivity than theselected superplastic metal.

(3) The mold or die is heated to a temperature substantially in excessof the critical forming temperature for the selected superplastic metaltaking into consideration such principles of heat transfer as relativesurface area to mass of the various mold configurations, the severity ofdeformation, particularly in small parts, the existence of an actualmelting limit and a desired final part temperature.

(4) Having thus selected a material, designed a mold and heated the moldto a predetermined temperature, the body of superplastic metal is placedin the mold where it can be heated from the die if desired orimmediately deformed. The temperature of the body of material rises asthe body receives heat from the mold. This temperature rise is arrestedlocally at the temperature of phase transformation due to the absorptionof heat required to effect the transformation. Formation should becompleted by the time that any full section of the superplastic materialhas been completely transformed.

(5) Preferably, the part thus formed is further heated by the die to atemperature definitely above the critical superplastic formingtemperature to assu", complete transformation of the metallurgicalstructure and the resulting increased strength necessary to enable itsimmediate removal from the die. In the case of the zinc-aluminumeutectoid, the additional heating prepares the formed material for aslow cool equilibrium phase transformation and grain growth and creepcharacteristics at room temperature.

These and other objects, features and advantages of our invention willbe apparent to those skilled in the art from the following descriptionof a specific application of our process wherein reference is made tothe accompanying drawings, of which:

FIG. 1 is a partially broken away perspective view of a molding diesuitable for performance of our process.

FIG. 2 is a front cross-sectional view of a portion of the die shown inFIG. 1 and including a part in place as molded therein.

FIG. 3 is a typical phase diagram describing a eutectoid phase phenomenaof the type that exists in the zinc-aluminum eutectoid.

Referring now to FIGS. 1 and 2, there is shown a mold, die or similarshaping member forming a cavity 11 that is complementary to theconfiguration of the part P desired to be formed. An eject pin or button12 is provided for assisting removal of the part P after it is formed.Heating means such as commercially available electrical resistanceheaters 13 are embedded in the body of die 10 and are separated from thecavity 11 by some thickness 14 of die material. If the die body 10 ismade of steel, for example, and it is desired to mold the zinc-aluminumeutectoid, the thickness 14 will determine the rate at which heat can betransferred from the heaters 13 to the body P being molded. Inasmuch asheat transfers nearly four times faster in the zinc-aluminum than insteel, the thickness 14 can effectively behave as a control on the rateof temperature rise of the outer surface of the body P. Heaters 13 areconnected to a suitable power source (not shown) through cable 15 andare maintained at a predetermined elevated temperature within relativelywide temperature limits.

A top plate or cover 16 is provided for enclosing the die cavity 11 andincludes an inlet opening 17 through which a blank or body of preparedstock superplastic metal P can be inserted. While the cover plate 16 isnot shown as being heated, it may be desirable to include heaterssimilar to 13 in the top plate, particularly where large parts are beingformed. In addition, top plate 16 can be made to include an insulatingmaterial particularly where a long forming process is involved.

In operation, top plate 16 is clamped or otherwise forceably held to thedie body 10 and the body of superplastic material P is placed in the diethrough inlet opening 17. Preferably, body P is preheated to atemperature close to the forming temperature. For example, in the caseof the zinc-aluminum having a critical forming temperature limit ofapproximately 532 degrees F., the body P may be preheated to withinapproximately 2% of the forming temperature, eg (taken on an absolutescale) between 500 and 520 degrees F., to minimize the requirements forheat transfer in the die itself. A plunger or piston 18 is then closeddown upon the body P and deforms the body by compression into intimatecontact with the cavity 11. The forming time is ideally selected suchthat complete formation just precedes complete transformation of thelast to be formed portions of the part P. The part P is left in the diecavity 11 for a short period of time (in the case of the zinc-aluminumthis period can be as short as five seconds for moderately small parts)to assure complete transformation throughout the body. The top cover 16is then removed from the die and the part P which is above the criticaltemperature can be ejected by force exerted upwardly against eject pin12. In the case of the zincalumin-um eutectoid, the part as ejected canbe slow cooled directly either in the air or under controlled conditionsin a heat treat furnace to increase grain size and to permit equilibriumphase transformation of the alloy.

The characteristics of a material suitable for use as the blank or stockP are illustrated by reference to a portion of the zinc-aluminum phasediagram, FIG. 3, which shows a typical equilibrium phase relationshipknown as a eutectoid. This specific eutectoid has a critical phasetransformation at the eutectoid invariant of 532 F. and has a whollysolid phase a up to temperatures as high as about 800 F. The a phase hasstrength characteristics like those of conventional metals Whereas the 4strength of this material just below the eutectoid invariant, when aproperly preconditioned state, is anomalously low.

The heat of phase transformation for this material is demonstrated by anexperiment as illustrated in FIGS. 4 and 5. Cylindrical slugs S ofquenched 78% zinc-22% aluminum were placed between heating platens 21and 22 as shown in FIG. 4. Steel stops 23 were also placed between theplatens 21 and 22 to eliminate significant deformation of the slug S. Athermocouple 24 was located in the center of the slug S and thetemperature change was recorded for a period of time during which theslug was elevated from below its superplastic critical temperature toabove that temperature. Platen temperature was varied between 550 and650 F. for dilferent tests and characteristic curves like curve S shownin FIG 5, were recorded in each instance. We noted that a level area ortemperature arrest S is a significant part of this temperature risecurve. The following data indicates the platen temperature for a slughaving a cylindrical conmagnitude of this temperature arrest relative tothe figuration with a diameter of two inches and a vertical height oftwo inches.

Duration of thermal arrest, secs.

Platen temperature degrees F ahrenheit:

EXAMPLE A typical part, as shown in FIG. 6, was repeatedly formed inaccordance with our invention. The part shown in FIG. 6 has an overalldiameter 31 of 2.520 in., a peripheral rim width 32 of .690 in., a hubdepth 33 of 1% in., a tooth height 34 of .055 in. and an overall weightof 177 grms. The blank of stock metal from which it was formed was adisk having a diameter of 2.36 in., a depth or thickness of .483 in. anda center bore of .343 in.

The blank is preconditioned by homogenization at 600 F. for one hourfollowed by a water quench with agitation. Low temperature recalescenceis permitted.

The blank is preheated at the time of forming to 500 E, which requires aperiod of about 2 minutes. A die mold of AISI Type H13 and A181 C10l8steels weighing 350 lbs. is heated by heaters totalling 6000 wattscapacity and separated from this die cavity by 1% in. at the closestpoint, to an initial temperature of between 610-630 F. These heaterssupplement the 29,300 watts available in press platens employed to clampthe die components together.

The blank disk was placed in the die which closes in a period of 5seconds. Forming occurs under a load that varies to a maximum of 60,000lbs. during a period of 4 to 7 seconds. The part thus formed is left inthe die under pressure for an additional 10 seconds to assure atemperature throughout in excess of 532 F. The cycle is completed byopening the die (approximately five seconds) and ejecting the part(three to five seconds). During this eight to ten seconds, approximately50% of the part remains in intimate contact with the 610630 F. lower diesection, thus subjecting the part to additional heat treatment. The partis air cooled to complete the combined forming and heat treatingprocess.

Our invention can also be applied to sheet forming techniques asillustrated in FIG. 7. A die or shaping member 40 like that described inaforesaid US. Pat. 3,340,101 is heated to above the critical temperatureof blank sheet metal B by heaters 41. A vacuum is applied to plenum 42to create a fluid pressure across the opposed principle surfaces B and Bof sheet B thereby deforming it into conformity with the die 40. Afterforming, the resulting part can be removed from the die 40 with littledanger of distortion.

Having thus described the concepts of our invention, some typicalapplications thereof and a specific performed example, we define theinvention sought to be patented by the following claims:

1. The method of molding solid metal of a metallurgical composition andstate characterized by conditionability to a state of anomalously lowstrength within a temperature range at and below a critical temperature,and the existence of a wholly solid phase having ordinary strength attemperatures above said critical temperature, comprising the steps of:

providing a shaping member defining a configuration to be molded,

providing a body of said metal at a temperature below said criticaltemperature,

heating said shaping member to a temperature substantially in excess ofsaid critical temperature, and deforming said metal body against saidshaping member to form a part having said configuration.

2. The method of molding as defined in claim 1 wherein said metal is ofa eutectoid alloy composition.

3. The method as defined in claim 2 wherein said conditionable metalcomprises, by weight, essentially 78% Zinc and 22% aluminum.

4. The method of molding metal as defined in claim 1 wherein said metalbody is provided at a temperature that is within approximately 2% ofsaid critical temperature.

5. The method of molding metal as defined in claims 1, 2, 3 or 4comprising the further step of separating said part from said shapingmember while said part is at a temperature in excess of said criticaltemperature.

6. The method as defined in claim 3 comprising the further steps of:

removing said part from said shaping member while said part is at atemperature in excess of said critical temperature, and

slow cooling said part to below said critical temperature at a ratesutficiently slow so as to permit substantially equilibrium phasetransformation of the metallurgical structure.

7. The process as defined in claims 1, 2, 3, 4, 5 or 6 wherein saidshaping member comprises a substantially enclosed cavity and saiddeforming is accomplished by application of compressive stress to saidmetal body.

8. The process as defined in claims 1, 2, 3, 4, 5 or 6 wherein saidmetal body is provided in a form having two opposed principal surfacesand said deforming step is accomplished by the application of a fluidpressure across said principal surfaces.

References Cited UNITED STATES PATENTS 1,993,942 3/1935 Novotny 264 -3282,814,101 11/1957 Prough et a1. 72342 3,340,101 9/1967 Fields, Jr., etal l48l1.5 3,420,717 1/1969 Fields, Jr. et al l4811.5

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examinermy UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,578 5 1 Dated ay 1 i i971 Mehi, Daniel L,; Wilson, Delbert T.; Young,Inventor(s) Richard J It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 4, i ine 2i deiete [magnitude of this temperature arrest reiativeto the] insert "figuration with a diameter of two inches and a verticaiColumn 4, line 22 delete [figuration with a diameter of two inches and averticai] insert "magnitude of this temperature arres t reiati ve tothe" Column 4, iine 27 delete [650-600] insert "650-660" Signed andsealed this 13th day of February 1973.

(SEAL) /\t test EDWARD M.I LETCHER ,JR. ROBERT GOTTSCHALK AttestingOfficer Commissioner of Patents

